Amorphous alloys (or glassy alloys) have been typically prepared by rapid quenching from above the melt temperatures to ambient temperatures. Generally, cooling rates of 105° C./sec have been employed to achieve an amorphous structure in these materials. However, at such high cooling rates, the heat cannot be extracted from thick sections, and, as such, the thickness of articles made from amorphous alloys has been limited to tens of micrometers in at least in one dimension. This limiting dimension is generally referred to as the critical casting thickness and can be related by heat-flow calculations to the cooling rate (or critical cooling rate) required to form the amorphous phase.
This critical thickness (or critical cooling rate) can also be used as a measure of the processability of an amorphous alloy (or glass forming ability of an alloy). Until the early nineties, the processability of amorphous alloys was quite limited and amorphous alloys were readily available only in powder form or in very thin foils or strips with critical dimensions of less than 100 micrometers. However, in the early nineties, a new class of amorphous alloys was developed that was based mostly on Zr and Ti alloy systems. It was observed that these families of alloys have much lower critical cooling rates of less than 103° C./sec, and in some cases as low as 10° C./sec. Using these new alloys it was possible to form articles of amorphous alloys having critical casting thicknesses of from about 1.0 mm to as large as about 20 mm. As such, these alloys are readily cast and shaped into three-dimensional objects using conventional methods such as metal mold casting, die casting, and injection casting, and are generally referred to as bulk-solidifying amorphous alloys (bulk amorphous alloys or bulk glass forming alloys). Examples of such bulk amorphous alloys have been found in the Zr—Ti—Ni—Cu—Be, Zr—Ti—Ni—Cu—Al, Mg—Y—Ni—Cu, La—Ni—Cu—Al, and other Fe-based and Ni-based alloy families. These amorphous alloys exhibit high strength, a high elastic strain limit, high fracture toughness, and other useful mechanical properties, which are attractive for many engineering applications.
Although a number of different bulk-solidifying amorphous alloy formulations have been disclosed in the past, none of these formulations contain a large amount of refractory metals, and as such they have limited high temperature stability. There is growing interest in developing bulk-solidifying amorphous alloys which have greater thermal stability and as well as higher strength and elastic modulus. Specifically, amorphous alloys which have a relatively high glass transition temperature, Tg, are of interest. These so-called “refractory” amorphous alloys could be used in a variety of high temperature applications presently unacceptable for traditional bulk amorphous alloys.
Accordingly, a need exists to develop bulk solidifying amorphous alloys with high temperature stability based on refractory metals.